What could this be?

My friend is caring for his elderly father who is in the very last stages of Alzheimers. The last few weeks his father has seemed much more alert and this morning he was even talking to the Nurse who comes to see them. However, this lunchtime he was leaning to the side, was complaining of being very tired and according to my friend his face had changed and he wasn't able to focus his eyes. He didn't want anything to eat and fell asleep while my friend was trying to feed him. My friend says that he is just suffering from mental tiredness after being so much better of late but I am not so sure. I wonder if anyone here might know what this could be? Should he call the Doctor? Last night his father also had constant diaorheaa, including some that was just like clear water. Please help me to help them both. I've tried to learn all I can about this awful disease but I just don't know what this is. By the way, I'm in the UK. Love Sarah